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THERE are 1.2 million people living in the Amsterdam area, 800,000 of them in the city itself. And the number is rising all the time.

The problem is there is just not enough housing. Consequently, there are plans to develop the area south of the river IJ into houses and offices. The IJ-boulevard will run through this area and then into the new crossing the Piet Hein Tunnel.

A light railway will run parallel to the road, servicing the new homes as well as a new urban development IJ-burg. About 40,000 people will be living in IJburg, which will be built on specially made artificial islands in the IJ-lake.

The need to build new houses had a great inuence on the design of the Piet Hein Tunnel. It could have risen to ground level earlier in its course, but keeping it below the surface allowed 1,000 more houses to be built.

An immersed tunnel design was chosen because the crossing passes through the Germany-Amsterdam shipping canal, says Stofful de Ronde,

project director for the clients designer and contract manager, SAT Engineering.

It was technically possible to design it as a cut-and-cover structure using sheet piles, says Mr de Ronde. But there is a law which says that the canal should be open all year, so we decided in 1990, at the pre-design stage, to make it immersed.

Sheet piles are being used in the sloping entrances to the tunnel. The piles form the side wall, the roof is concrete and the existing clay forms the oor. This is known as polder construction.

But this method has had to be reinforced at the east entrance to the tunnel, where weak ground meant solid foundations had to be installed.

The poor ground conditions meant the service building at the eastern end required 60 m long piles, rather than the 20 m ones designed initially. These reinforcements to the design added 15 million guilders (6 million) to the price.

The project is being paid for jointly by the city of Amsterdam and a 150 million guilder subsidy from the government.

Amsterdam is hoping for more government money to help build the railway which, initially, will be a light rail system. However, it may be upgraded to a metro later.

In fact, the railway has not yet been given the go-ahead, but it made sense to make provision for it within the tunnel because building one later would be three times more expensive.